1. Field of the Invention
The present invention relates to a motor control apparatus for an electric power steering system by which apparatus a motor-driven auxiliary steering power (power assist) is given to the steering system of a vehicle.
2. Related Background Art
A motor control apparatus for an electric power steering system of this type has already been proposed by the present applicant (refer to Japanese Patent Laid-open Publication 61-98675). This motor control apparatus is provided with a torsion torque sensor for detecting a torsion torque of the steering system as well as a vehicle speed sensor for detecting a vehicle speed and a steering angle sensor for detecting a steering angle. A command signal obtained as an output from the torsion torque sensor decreases as the vehicle speed obtained increases. The command signal is added to a return signal which increases as a steering angle obtained as an output from the steering angle sensor increases. The added signal is used for controlling the rotational direction and assist torque of the electric motor of the power steering system.
With the conventional motor control apparatus for the electric power steering system described above, power assist during a straight forward running of the vehicle has been (neglected) considered unnecessary. In this system a dead zone has been provided within a range of a small torsion torque. Accordingly, the power assist has not been obtained when a driver is holding a steering wheel for running on a gentle curve or when a driver is correcting the steering wheel after turning it, leaving the power assist function unsatisfactory.